The factors in liver cytosol which affect coenzyme dissociation of pyridoxal-P from tyrosine aminotransferase are under investigation. There appear to be at least two factors other than the architecture and composition of the coenzyme binding site, a macromolecule and a micromolecule. The activity of the micromolecule can be expressed by Mg ion 2. Somatic cell genetics is being used to determine the nature of the expression of tyrosine aminotransferase in cells in culture. The expression of liver phenotypes, including this enzyme, are suppressed when rat hepatoma cells are hybridized with mouse fibroblasts or with mouse teratocarcinoma cells.